To measure various processes, process measuring devices are known in which a measuring sensor is placed in a process medium flow channel or the like, and the actual measuring means is placed outside this flow channel. In such a process measuring device, a sensing element is fixed to a lever-like transfer element for transmitting the force and/or movement exerted by the process medium on the sensing element by the torque principle to the measuring means outside the flow channel system. The measuring means is a force transducer for measuring the force dependent on the torque applied to the transfer element constituting the lever arm, the force being proportional to a variable to be measured, such as consistency of the process medium. The process medium flow channel is provided with a measurement coupling comprising an opening through which the lever arm is introduced. The process measuring device can thus be divided into a portion on the process side and a portion outside the process. A diaphragm is placed in connection with the opening in the measurement coupling to prevent the process medium from entering into the portion of the process measuring device that is located outside the process. However, because the torque effect of the force applied to the sensing element must be transmitted by the lever arm to the portion of the process measuring device outside the process, this diaphragm must be made as elastic as possible, so as not to cause distortions in the measuring result on one hand and not to make the measuring device too insensitive on the other hand. Furthermore, it is not desirable that the measuring result be affected by other factors than the variable to be measured.
The diaphragm is subjected to very high demands, because if entered in the portion of the process measuring device that is outside the process, the process medium may cause serious damage to the process measuring device and may even break the process measuring device. Thus, the process measuring device must be either repaired or replaced by a new one, which will in any case cause extra costs and may even require a stoppage of the process until the damaged process measuring device has been repaired or replaced by a new one.
Finnish patent 75424, to which corresponds e.g. U.S. Pat. No. 4,757,708, presents one inlet diaphragm construction. The diaphragm is fixed to the frame of the process measuring device on one hand and between the first and second rod parts of the lever arm on the other hand. Thus, the diaphragm covers the opening of the measurement coupling. The first rod part is intended to be placed on the process medium side in the flow channel and, correspondingly, the second rod part is intended to be placed outside the process medium flow channel. A sensing element if fixed to the first rod part. The first and second rod parts are attached to each other preferably with a threaded joint, wherein either of the rod parts is arranged to penetrate the opening formed in the diaphragm to make the threaded joint between the rod parts and to clamp the diaphragm between the first and second rod parts. In some phases of the process, such as in starts, so-called clods or other accumulations may pass in the flow channel and hit the sensing element so strongly that the first rod part bends. For this reason, existing measuring devices are equipped with a so-called mechanical fuse which is produced by making the rod part thinner. In spite of this mechanical fuse, this impact-like force effect may cause a slight opening of the joint between the first rod part and the diaphragm, allowing process medium to enter inside the process measuring device, and serious damage may consequently be caused.